Cleanup registering and trusting types for serialization

[ktoso]

Some of the names are a bit weird with registerInbound and there's some overlap with trusting error types.

We should make some settings.serialization.trust(Type.self) or similar maybe?

[ktoso]

Also cleanup this:

        public var insecureSerializeNotRegisteredMessages: Bool = _isDebugAssertConfiguration() // TODO: We are using an internal function here to allow us to automatically enable the more strict mode in release builds.